1. Field of the Invention
The present invention relates to high frequency switchmode power inverters and specifically to high frequency electronic ballasts for gas discharge devices. More specifically, the present invention relates to a high frequency electronic ballast for high intensity discharge (HID) lamps.
2. Prior Art
The prior art is replete with many known circuits providing high frequency ballast for gas discharge lamps, especially for high intensity (HID) lamps. For instance, high efficient electronic ballasts which can be used with HID (HPS) lamps are U.S. Pat. No. 5,097,183, entitled "Master-Slave Half-Bridge DC-to-AC Switchmode Power Inverter" and U.S. Pat. No. 5,229,927, entitled "Self-Symmetrizing and Self-Oscillating Half-Bridge Power Inverter", assigned to the assignee of the present invention.
An important application of switchmode power inverters is supplying gas discharge devices, especially high intensity discharge (HID) lamps in the range of 35W to 400W. In this case, the load impedance of the inverter is a HID lamp connected in series with an inductor. In the case of a high frequency powering of the HID lamp, the interaction between the high frequency ballast and the lamp is stronger than that of a conventional low frequency (50/60 Hz) ballast. The high frequency ballast can be significantly better than a conventional low frequency (50/60 Hz) ballast due to its lessened weight and essentially high efficiency. Additionally, the high frequency ballast, utilized with an HID lamp would have a simple, reliable dimming capability and an essentially more reliable light switch (day and night switch) than the conventional low frequency ballasts.
Therefore, the critical design targets for high frequency ballasts supplying HID lamps would be the following:
a) very high efficiency, &gt;94% (energy saving and low temperature rise of the critical components); PA1 b) fully controlled lamp power eliminating the effects of input voltage fluctuation and the lamp aging; PA1 c) dimming capability, from 100% to 50% (35%); PA1 d) automatic light switching (day and night time switch); PA1 e) over temperature protection; PA1 f) automatic switch off in the case of lamp failure; PA1 g) instant re-ignition of the warmed up lamp; and PA1 h) isolated interface circuit providing the connection of several ballasts and therefore implementing fully controlled lighting system.